1. Field of the Invention
This invention relates to a composition comprising silane crosslinkers and two or more free radical initiators with different half-life temperatures and a process of crosslinking thermoplastic polymers with silane crosslinker in the presence of a blend or mixture of free radical initiators. The invention further relates to the moisture crosslinked thermoplastic polymers resulting from the process.
2. Description of Related Art
For many applications, e.g., wire and cable insulation, weatherstripping, fibers, seals, gaskets, foams, footware, flexible tubing, pipes, bellows, tapes, etc., certain selected properties (e.g. tensile strength, compression set, thermal and chemical resistance) of articles manufactured from one or more thermoplastic polymers can be enhanced by introducing chemical linkages between the polymeric molecular chains which constitute the polymer, during or preferably following, the shaping or molding process. These chemical linkages between different polymeric molecular chains are commonly referred to as “crosslinks”. Crosslinks can be introduced between different molecular chains of a thermoplastic polymer by a number of mechanisms, one of which is to graft to the individual polymer backbones or chains that constitute the bulk polymer with a chemically reactive compound in such a manner that the grafted compound on one backbone may subsequently react with a similar grafted compound on another backbone to form the crosslink. Exemplary of this process is the “silane crosslinking” process.
This process employs a silane-containing compound that crosslinks these thermoplastic polymer compounds. Silanes can be grafted to a suitable thermoplastic polymer by the use of a suitable quantity of organic peroxide or other free radical initiator, either before or during a shaping or molding operation. Additional ingredients such as stabilizers, pigments, fillers, catalysts, processing aids etc., may also be included in the mixture.
When using silane-peroxide blends for thermoplastic polymer crosslinking, a compromise must be made between grafting efficiency and process efficiency, such as extrusion rate and run times. The formation of a cross-linkable material by this means is, however, difficult to carry out since it requires critical control of the process. If the process is carried too far, the thermoplastic polymer may partially cross-link and solidify in the processing apparatus, for example an extruder, with consequent difficulties in achieving consistent good quality products and delays involved in removing the partially cross-linked product from the processing equipment. Care must also be exercised to ensure that articles prepared from the polymer retain their shape during subsequent heating to bring about the cross-linking process.
It has been observed that gel formation, screw-build up and scorching may result when using highly reactive silane-peroxide blends. This is particularly significant for processes using conditions and processing equipment that impose severe melting and mixing conditions leading to high shearing stresses in the polyolefin. These problems generally arise due to early and eventually complete activation of the peroxide during the initial melting and homogeneisation process. The prior art has dealt with these problems by using less reactive silane blends but this approach can diminish the grafting efficiency of the crosslinkable thermoplastic polymers.
U.S. Pat. No. 3,646,155 describes the crosslinking of polyolefins by the reaction of polyolefin with an unsaturated hydrolysable silane at a temperature above 140° C. in the presence of a compound capable of generating free radical sites in the polyolefin.
U.S. Pat. No. 4,252,906 describes a process using a crosslinkable polyethylene resin composition comprising a silane-modified polyethylene resin and an organic peroxide.
U.S. Pat. No. 4,117,195 describes what is commercially known as the monosil process of making a crosslinked extruded product from polyethylene or other suitable polymer.
U.S. Pat. No. 4,412,042 describes a process for preparing polyolefins cross-linked by a silane linkage. This is accomplished by reacting a silane with an ethylene/α-olefin copolymer of a specific density in the presence of a free radical generating agent, e.g., a benzoyl peroxide.
U.S. Pat. No. 5,112,919 describes what is commercially known as the XL-Pearl process involving the solid feeding of silane cross-linking agents into an extruder.
U.S. Pat. No. 5,741,858 discloses a silane-crosslinked blend comprising a polyolefin elastomer, a crystalline polyolefin polymer, a silane crosslinker and preferably a peroxide initiator.
U.S. Pat. No. 5,744,553 describes what is commercially known as the spherisil process.
U.S. Pat. No. 6,455,637 discloses a coating material formed by the process of reacting a polyolefin with a silane in the presence of a free-radical initiator.
EP 1288235 describes a composition for cross-linked polymers that exhibit scorch and gel formation reduction properties. This reference is incorporated by reference herein.
There remains a need for a means of crosslinking polyolefins and other silane crosslinkable thermoplastic polymers under reactive mechanical-working conditions using silane crosslinkers and free radical initiators while minimizing such aforenoted problems as gel formation, screw-buildup and/or scorching while maintaining a high level of grafting (crosslinking) efficiency.